From CSS 2.1, http://www.w3.org/TR/CSS21/fonts.html#font-family-prop
# Font family names must either be given quoted as strings,
# or unquoted as a sequence of one or more identifiers. […]
#
# If a sequence of identifiers is given as a font family name,
# the computed value is the name converted to a string by joining
# all the identifiers in the sequence by single spaces.
Identifiers tokens can be separated by comments, whitout whitespace.
For example, 'font-family: Fontin/**/Sans' should parse the same as 'font-family: Fontin Sans',
but doesn’t with a recent mozilla-central build.
I have a patch to fix this, but I’m adding some tests before submitting.
The patch only affects ParseOneFamily in nsCSSParser.cpp
and should apply independently of bug 280443.

Comment on attachment 753618[details][diff][review]
Proposed fix, with a reftest
Seems like a good test to contribute to the CSS 2.1 or css3-fonts test suite.
Also, perhaps rename the test to font-family-whitespace-1 or something like that? (I dislike bug numbers in test filenames.) That's easy to fix by search-replacing the patch.
r=dbaron